1. Field of the Invention
This invention relates to the recovery of coal combustion products from landfills, and more particularly to a process for recovering coal ash from landfills and reburning the coal ash in a coal fired boiler.
2. Description of the Related Art
Coal fired boilers are widely used to generate steam for producing electricity. A common form of boiler uses a pulverized coal that is injected into a furnace. Millions of tons of coal ash result each year from such operations. The coal ash includes fly ash with a minor proportion of bottom ash. Some of the coal ash is commercially usable in concrete, concrete products, cement production, sewage sludge stabilization, pavement base materials, lightweight aggregate, and other miscellaneous purposes. The remaining coal ash must generally be disposed of by landfilling since it has no commercial value.
A principal reason for a lack of commercial value for coal ash is the presence of unburned carbon in the ash. (When using the term xe2x80x9ccarbon content in ashxe2x80x9d, the carbon content is as measured by loss on ignition.) According to ASTM (American Society for Testing and Materials) C 618 xe2x80x9cStandard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concretexe2x80x9d, a fly ash must have a loss on ignition (LOI) value no higher than 6% for use in concrete. An upper limit of 3% LOI is more realistic. Higher loss on ignition ash cannot be used because of color problems and concerns for durability under freezing and thawing conditions. The residual carbon content in the coal ash depends upon a variety of factors including base line furnace operation and boiler design. It also depends upon the source of the coal fuel. For example, subbituminous western coal, when burned, will typically result in a low carbon ash with a loss on ignition of less than 5%. However, when bituminous coal is burned, a higher carbon ash with a loss on ignition significantly greater than 5% is generally produced as a byproduct.
Because of the widespread use of coal fired boilers to generate steam for producing electricity, it can be appreciated that significant volumes of coal ash have been deposited in landfills over the years, particularly in regions where electricity was or is still being generated by burning bituminous coal in a coal fired boiler. In addition, the use of greenhouse gas emission technologies, such as low NOx burners, have a tendency to increase the loss on ignition of a coal fired boiler""s fly ash and thereby make the fly ash unusable because of high carbon content (e.g., LOI greater than 6%). As a result, a coal fired boiler using greenhouse gas emission technologies will also produce coal ash that must be landfilled. It is also well known that landfill space is rapidly dwindling in many regions and that the construction of new landfills is very costly and, in some regions, is not even an option because of the scarcity of available landfill sites.
Accordingly, there is a need for a process that can recover coal combustion products from a landfill in order to free up landfill space for the disposal of other products, and thereby eliminate the need for the construction of additional landfills. More particularly, there is a need for a process that can recover coal combustion products from a landfill in order to free up landfill space and that can cost effectively produce commercially usable products such that the process is economically attractive.
The foregoing needs are met by a process according to the invention wherein a disposal site that contains coal combustion products is identified, the coal combustion products are recovered from the disposal site, and the recovered coal combustion products are reused. In one version of the invention, the process comprises using the coal combustion products removed from the disposal site as an aggregate for construction projects. In another version of the invention, the step of reusing the coal combustion products comprises: introducing pulverized coal into a pulverized coal fired furnace; introducing the recovered coal combustion products into the pulverized coal fired furnace; and burning the recovered coal combustion products in the furnace along with the pulverized coal in a manner such as that described in U.S. Pat. No. 5,992,336 and PCT International Publication Number WO 98129687, which are incorporated herein by reference.
In yet another version of the invention, the step of reusing the coal combustion products comprises: analyzing the recovered coal combustion products to determine the loss on ignition of the recovered coal combustion products; introducing pulverized coal into a pulverized coal fired furnace, the pulverized coal typically being subbituminous coal or other coal which generates low loss on ignition ash upon burning; introducing the recovered coal combustion products into the pulverized coal fired furnace if the recovered coal combustion products have a loss on ignition greater than or equal to about 5%, and reburning any recovered coal combustion products introduced into the furnace with the pulverized coal (the recovered coal combustion products typically being in the range of 0.5% to 3.5% by weight of the pulverized coal) to produce a coal ash having a low loss on ignition. In this version of the invention, landfilled coal combustion products which have a loss on ignition greater than or equal to about 5% (and therefore, limited commercial value) are reburned to produce a coal ash which has a loss on ignition less than about 3% (and therefore commercial value in concrete). As a result, this version of the invention provides a number of key advantages. For instance, the invention: (1) frees up landfill space; (2) transforms coal combustion products with no commercial value into coal ash with significant commercial value; (3) recovers lost energy remaining in the coal ash; and (4) removes potential future environmental risk associated with leaching from non-state-of-the-art landfills and ash storage ponds and impoundments.
In still another version of the invention, the step of reusing the coal combustion products comprises: analyzing the recovered coal combustion products to determine the loss on ignition of the recovered coal combustion products; introducing pulverized coal into a pulverized coal fired furnace, the pulverized coal typically being subbituminous coal or other coal which generates low loss on ignition ash upon burning; introducing the recovered coal combustion products into the pulverized coal fired furnace if the recovered coal combustion products have a loss on ignition greater than or equal to about 1%, and reburning any recovered coal combustion products introduced into the furnace with the pulverized coal (the recovered coal combustion products typically being in the range of 1% to 3.5% by weight of the pulverized coal) to produce a coal ash having a low loss on ignition.
It is therefore an advantage of the present invention to provide a coal combustion products recovery process that removes and recovers coal combustion products from a landfill and characterizes the recovered coal combustion products so that the recovered coal combustion products may be put to beneficial use.
It is another advantage of the present invention to provide a coal combustion products recovery process that serves to preserve licensed landfill airspace.
It is still another advantage of the present invention to provide a coal combustion products recovery process that reduces the need for new gravel pits, stone quarries and landfill sites.
It is yet another advantage of the present invention to provide a coal combustion products recovery process that allows for the recovery and reburning of high loss on ignition landfilled coal combustion products, such as formed from a pulverized coal furnace burning bituminous coal, to render the coal combustion products into a commercially valuable fly ash and bottom ash having very low loss on ignition.
It is another advantage of the present invention to provide a coal combustion products recovery process that preserves present coal reserves by recovering heat, as commonly measured by British thermal units (Btu), from reclaimed coal combustion products.
It is still another advantage of the present invention to provide a coal combustion products recovery process that produces a low loss on ignition coal ash that may be substituted for portland cement thus offsetting additional portland cement production and thereby reducing air emissions from the portland cement production process.